The control of normal movements by the nervous system involves accurate sequencing and modulation of muscular contractions. The 'motor programs' for certain stereotyped movements are, to some extent, intrinsic to the spinal cord, but may be modified depending upon the progress of the actual movement. The long-term objective is to elucidate the rules whereby the attributes of actual movement, as signaled by the activity of afferent nerve fibers from peripheral receptors, serve to modify the timing and strength of muscular contractions. The significance of this objective lies in providing a framework for the understanding of the mechanisms that underlie movement disorders, and also in the design of orthotic and prosthetic devices capable of responding to the vicissitudes of the environment. The influence of afferent activity is proposed to be investigated in spinalized turtles; a lower vertebrate is chosen for the technical feasibility of the various recordings. Three specific issues are addressed. 1. Can the effects of afferent activity be described in terms of resistance to, and assistance of, external perturbations? 2. Are muscles utilized to compensate for the insufficiency of action of their anatomical synergists, or must the contractions of synergists co-vary? 3. What effects of afferents underlie the surprising adequacy of performance when the inertia of the moving limb is increased? The methodology for achieving these aims relies upon the ability to monitor electromyographic activities, muscle length, and a sample of activity of muscle spindle afferents, during ongoing movement. The movement, elicited by gentle, mechanical stimulation, is the scratching or wiping movement of a hindlimb. Mechanical perturbations are applied, and their effects assessed in relation to the effects observed after selective deafferentation and/or tenotomy.